1. Field of the Invention
This invention relates to a magnet apparatus for use in a magnetic resonance imaging system (MRI system) and more specifically to a magnet apparatus having a cryostat to keep a superconducting coil, which produces a static magnetic field, in the superconducting state.
2. Description of the Related Art
The MRI system has formed therein a bore to accommodate a patient. A working volume where the patient's part subject to diagnosis is placed is defined in the bore. A magnet apparatus is disposed surrounding the bore and comprises a superconducting coil to produce a static magnetic field in the working volume and a cryostat to hold the superconducting coil in the superconducting state. A gradient magnetic field is superposed on the static magnetic field and high frequency signals are applied to the patient's part subject to diagnosis. Magnetic resonance signals from that part of the patient are detected and used to form a tomographic image of the part for diagnosis.
Incidentally, the static magnetic field formed in the working volume is required to be high homogeneity. This has caused the superconducting coil to be made with a large volume, which led to the increased volume of the cryostat. Thus, the production costs of superconducting coils and cryostats have been pushed up substantially. As superconducting coils and cryostats are great in size and weight it is difficult to transport MRI systems and install these systems in the existing hospitals or the like.
Further, the whole body of the patient is accommodated in the long and narrow bore. Therefore, the patient has an oppressive sensation and often suffers from claustrophobia. While MRI diagnosis is in progress, the doctor cannot directly observe the patient. If, under such a situation, the patient's condition takes a sudden turn for the worse, the doctor is unable to take an emergency measure.